The present application claims the benefit of Japanese Patent Application No. 11-374449 which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a stereomicroscope and a dark field illumination apparatus used therein.
2. Related Background Art
When observing a subject with a vertical illumination and, a transmission illumination by use of a stereomicroscope, if the subject is a transparent living specimen and a hyperfine structure of a flaw etc of a jewel, there might be a case where a contrast with a background is hard to obtain and the hyperfine structure is hard to distinguish. In such a case, a dark field illumination is used. The dark field illumination involves the use of only scattered light and diffracted light due to the subject as observation light without letting illumination light beam directly incident on an observation optical system, and a periphery of the subject is made to become a dark background, whereby images easy to observe the transparent subject and the hyperfine structure can be obtained.
What is known as a conventional dark field illumination apparatus is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 61-102618, wherein a light source is disposed at a center of a barrel type toroid mirror, and a light scattering structure is disposed between the barrel type toroid mirror and the light source. Further, Japanese Patent Application Post-Exam Publication No. 5-16567 discloses a dark field illumination apparatus in which an annular reflective member disposed along a periphery of a light source, and numerous number of hairlines are cut in this annular reflective member in a direction parallel to an optical axis. Moreover, Japanese Patent Application Laid-Open Publication No.11-153755 discloses a dark field illumination apparatus in which an annular reflective member is disposed along a periphery of a light source and is formed with a multiplicity of cylindrical curved surfaces extending in a direction parallel to an optical axis. Each of those apparatuses attempts to attain the illumination with no ununiformity by irradiating the sample with the light beam emitted from the light source, which is scattered by the light scattering structure, the hairlines and the cylindrical curved surfaces.
Each of the conventional dark field illumination apparatuses is constructed to illuminate the subject with the scattered light, and yields an advantage that the ununiformity of the illumination is hard to occur. While on the other hand, only a portion of the light beam emitted by the light source falls on the subject, and hence a quantity of the illumination light incident on the subject does not become so large. Consequently, when trying to observe a subject exhibiting small efficiencies of scattering and diffracting the light by the conventional scatter-type dark field illumination, quantities of the scattered light and of the diffracted light are small, with the result that only a dark image is obtained and hard to observe.
It is a primary object of the present invention, which was devised to obviate the above problems inherent in the prior art, to provide a stereomicroscope having a dark field illumination apparatus capable of irradiating a subject with the light emitted from a light source at a high efficiency and with less ununiformity of illumination.
To accomplish the above object, according to one aspect of the present invention, a stereomicroscope comprises an objective lens, a subject placing surface for placing a subject, and a dark field illumination apparatus disposed in such a position as to face to the objective lens with the subject placing surface being interposed therebetween. The dark field illumination apparatus includes a light source disposed on an optical axis of the objective lens, an annular reflective member for reflecting the light from the light source by its inside surface, and a light shielding plate for cutting off partially light reflected by the annular reflective member. The inside surface of the annular reflective member takes such a configuration that a plurality of reflecting surfaces each taking a concave shape having a predetermined radius of curvature are disposed in an annular shape with the optical axis being centered within a plane perpendicular to the optical axis, and the optical axis is positioned between the plurality of reflecting surfaces and centers of the curvatures thereof.